If I had one of those stainless power inlets I'd probably paint it black so it was less visible. I like to keep it clean looking on the outside. Besides, I thought the 50 amp 110 volt plug was expensive. The 50 amp 240 volt power inlet is about double the cost of the first inappropriate but still quite expensive one.

Seriously, they would produce a 50 amp, 110 volt power inlet? I'm betting they don't take returns.

The 50amp plug-in I want to install on the outside is going to be permanent and will stay where I put it, I don't want to put more holes in the outside skin then I need to, so I won't be moving it. But on the inside I'll be tearing out the bus wall and floor then insulating and building the inside. Once I get the inside framed in I'll be permanent mounting an "inverter-converter-charger" plus main power panel So I'll have the power I want. HOWEVER for now I just want a small movable fuse box inside so I can have power to run my tools Most people I know call it a "spider power box" that is what will be temporary for the near future.

772-I am not an electrician and didn't see any links for yours but I do know the many commercial spider boxes that I have worked around 1- have a recessed twist lock connector for all connections
2- have an internal breaker/GFCI panel
and for me I have seen do admiral jobs fully exposed to the rain, sitting in mud holes, hanging in buildings with no roof/upper floors with leaks and wide open to the elements?
I would say choose whether you want in the end to mount it underneath or inside?
In my opinion either way I would penetrate the floor of my electrical area and not the side skin/wall of the bus because the side Is just like putting a hole in the side of your house it is going to see more exposure to the elements than the underside especially if you build a sealed electrical compartment?
Any electrical compartment does need to breath including inside and please don't put batteries inside.
The many electrical suppliers around make connectors, boxes and all kinds of things outdoor electrical. I know the connectors favored for the GOVT work that I do a called LIQUID TIGHT/LIQUA TIGHT and you can get just about anything you want? Just know what size SO/SJ cord (indoor/outdoor extension cord) you want and they have a connector for it
I get to scrounge and salvage in what I do and for now I have 10 guage extension cords 75-100 long ran through my floor (I used a liquid tight connector in and out and a coupling tight to the underside of the floor with some caulking) that way whatever campsite I go to I can plug in and the length is because when I have to run my old generator I can get it as far away as possible because it is old and loud and if it messes up I can fix it and have him running in 10-15 because we have known each other for awhile and if it does get permanently mounted it will be with lots of isolaters and am considering putting on isolation spring mounts period and it will have a car/truck muffler affixed to it?
I can only give my opinion and I have done a form of your spider box but on a smaller scale and shore power ran everything except for an electric heater last winter so I ran a second cord.
Shore power was fine but the GFCI on my smaller 8 plug name brand power strip didn't like it ?
I understand and agree with starting with what you want.
I put a 2" connector in my floor that should be able to handle everything from my battery compartment and dedicated 120 cord storage from underneath?
I know the true sparky's are going to start hollering about if the 120 and 12 are together in the same conduit you will get feedback/residual voltage?
Heard it many times and in full operation of my setup (no I don't have a converter/inverter in play yet? But isn't that what they do?)
I can measure switch on off 120 on off and can't find it? And I didn't use shielded wire? One of my sub's /controls uses 24vshielded cable 20guage because of Ethernet/bacnet (computer stuff ) and it is not common practice to mix any voltages but they are paid to run specific things to specific points.
Sorry for the long message
What you do is up to you .
Instead of the big box store? Look into commercial electrical suppliers in your area. They are there are there not that far away ? And for some things they will have in stock and others they have a full catalog that you can sit in front in there chair and pick and choose/ask questions and order what you want or say sorry I have a different idea???

I'm also stripping a bus and needing a more permanent source of power than extension cords. I'm just going ahead with the permanent small breaker box. I was told I need a branch box with separate neutral and ground bars. I'm not finding anything like that at the home store.
Why go temporary anyway?

Try asking for a "sub-panel" or a "main lug load center." This one on Home Depot's web site has only one bus bar. It looks like it's electrically isolated from the steel enclosure (but do verify the fact). That bus bar could then be used for the neutral, and you can buy separately an extra bus bar to mount inside the box for terminating the ground wires. (an extra insulated neutral bar is also available, if needed.)